The proposed research is directed towards a better understanding of the mechanisms involved in the regulation of cholesterol biosynthesis. In particular these efforts will focus on the physiological alterations in 3-hydroxy-3-methylglutaryl coenzyme A reductase, the enzyme which catalyzes the rate-limiting reaction of cholesterol biosynthesis. The question of whether dietary and hormonally induced changes in reductase activity are due to changes in the number of enzyme molecules or to changes in the catalytic efficiency of the enzyme will be examined by enzyme purification and immunotitration. In order to gain some insight into the alterations which might be responsible for observed changes in the catalytic efficiency of the reductase, the properties of the enzyme purified from rats of different dietary and hormonal states will be compared. Properties to be examined include: affinity for substrates, presence of isoenzymes, activation energies, transition temperatures, pH optimums and aggregation - disaggregation. In addition, the possible role of lipids in changes in the catalytic efficiency of the reductase will be studied.